The present disclosure relates to a secondary battery, a manufacturing method of a secondary battery, an electrode for a secondary battery, and an electronic device. More particularly, the present disclosure relates to, for example, an electrode suitable for use in an all solid-state lithium (Li) ion battery and the like, and a manufacturing method thereof; a secondary battery such as a lithium ion battery using this electrode, and a manufacturing method thereof; and an electronic device using the secondary battery.
In recent years, an all solid-state lithium ion battery using a solid electrolyte which is a lithium ion conductor attracts attention, which acts as a secondary battery having a higher safety compared to an existing lithium ion battery using, as an electrolyte, a non-aqueous electrolyte in which a lithium salt is dissolved into an organic solvent. That is, the lithium ion conductor constituting the solid electrolyte is a single ion conductor in which only a lithium ion moves so that a side reaction and deterioration of an electrode accompanied thereby hardly take place compared to a secondary battery using a liquid electrolyte. Accordingly, the all solid-state lithium ion battery is a promising entry for a battery for an electric vehicle and a large-sized rechargeable battery.
In particular, the all solid-state lithium ion battery is expected to be preferably used as an in-vehicle higher output electric source because it is highly functional, highly reliable, highly risk-free without liquid spill, can obtain clean energy, is light-weighted, and can obtain higher energy density.
Among the all solid-state lithium ion battery, one of the most promising batteries in terms of practical realization is an oxide all solid-state lithium ion battery in which all of a positive electrode, a negative electrode and an electrolyte constituting the battery are constituted by a chemically stable oxide such as oxide ceramics.
As one of the methods of manufacturing the all solid-state lithium ion battery, a manufacturing method of laminating green compacts has been proposed (for example, see Patent Literature 1).
Although an oxide has excellent chemical stability, it has low ion conductivity on the other hand. Furthermore, an electron hardly passes through in a particle boundary between neighboring particles. For this reason, the oxide all solid-state lithium ion battery has had a problem that the impedance is large as a whole. It is generally known that among these problems to be solved, the ion conductivity can be improved by mixing a solid electrolyte in an electrode to generate an ion conduction path. However, when the solid electrolyte is mixed to the electrode too much, the ratio of an electrode active substance in the electrode decreases, and furthermore, the contact interface between the materials also increases. Accordingly, the electric resistance in the above particle boundary increases. Therefore, improvement in conductivity cannot be expected. In this manner, the improvement of ion conductivity and the decrease of the electric resistance in the particle boundary are in the relationship of trade-off. Therefore, there has been a limit in improving the ion conductivity and decreasing the impedance of the whole battery only by mixing the solid electrolyte. Therefore, the existing oxide all solid-state lithium ion battery has had a problem that a so-called rate property is low, in which the charge and discharge with a large electric current is difficult.
To address this concern, from the viewpoint of minimizing the influence of the above problems, a thin film oxide all solid-state lithium ion battery having a very thin electrode has been mainly proposed so far. However, when the electrode is thinner, the amount of the electrode active substance in the electrode is also naturally reduced. Accordingly, there is a limit in the rate properties that can be realized. Furthermore, there has been proposed that a sulfide having ion conductivity higher than an oxide is used as the solid electrolyte to improve the rate properties. However, a sulfide has a problem in chemical stability, and it could not be said that the charge-discharge cycle properties of the obtained all solid-state lithium ion battery are good. From the above, in the existing all solid-state lithium ion battery, the high rate properties and the favorable charge-discharge cycle properties were difficult to be mutually compatible (for example, see Patent Literature 2).
To address this concern, in order to reduce the above resistivity in the particle boundary, sintering at high temperature when manufacturing the oxide all solid-state lithium ion battery is generally performed. By performing the sintering, physical and electrical bonding properties between particles are improved, and an electron becomes likely to pass through the interface between the mutually neighboring particles. However, when the material undergoes this sintering process, it is subject to a change in substance and the like. Accordingly the chemical stability is impaired compared to before the sintering process. Thus, there has been a problem that deterioration in charge-discharge cycle properties due to use becomes larger (for example, see Patent Literatures 3 to 5).